between shadows and souls
by bravevulnerability
Summary: "The elusive 3XK killer, Richard Castle, comes out of hiding after years of solitude and becomes attracted to the NYPD's greatest homicide detective Detective Kate Beckett. His attraction becomes a game and she takes the bait, finding herself slowly involved in his world and unable to shake him." AU. One shot.


_Prompt from a tumblr anon: "The elusive 3XK killer, Richard Castle, comes out of hiding after years of solitude and becomes attracted to the NYPD's greatest homicide detective Detective Kate Beckett. His attraction becomes a game and she takes the bait, finding herself slowly involved in his world and unable to shake him."_

* * *

 **A/N: Obviously, this is very much an AU scenario.**

* * *

 _"_ _I love you as certain dark things are to be loved,_

 _in secret,_

 _between the shadow and the soul."_

 _-Pablo Neruda_

* * *

"You let me catch you."

Castle pauses, glancing up from his phone to see her standing within the mouth of an alley, a former crime scene for one of his kills. He sighs, tucks the device into his coat pocket.

He called her at work, taunting her, texting her a cryptic riddle like a clue to a treasure map.

 _Come and find me, Kate._

"Detective Beckett-"

"You _wanted_ me to," she hisses, striding up to a serial killer in a dark alley. But she isn't afraid, not of him. Not when it comes to her; he would never touch her. "What kind of sick game-"

"It's not a game," he argues and she balls her fists at her side.

"Everything is a game to you," Kate growls, shoving on his chest instead, watching him take it. "The lives you take, the collateral damage you cause, my career-"

"This," he murmurs, regaining his balance to step up to her. The imposing height, the breadth of his upper body, towers over her, but she merely grinds her teeth. "Is not a game. You, Kate Beckett, are the first thing that has been serious to me in nearly a decade."

"Murder isn't serious enough for you?" she snarls, glaring up at him in the darkness. "These are innocent women-"

" _Innocent_?" he snaps in disdain. "You profiled me as a sociopath, right? A killer without cause who just goes around plucking blonde women off the streets? You don't know anything about me."

"I don't _want_ to know you. All I know is that six women are _dead_ because of you."

"Then why are you here?" he challenges, one of his eyebrows arching. "You didn't come to arrest me, you're not calling backup, so what? If I'm such scum to you, why did you even waste your time tonight?"

Beckett purses her lips, her nostrils flaring, and takes a step back.

"Tell me why you killed them, why you do any of this," she demands, crossing her arms over her chest. "If you're not just some killer without a cause, prove it to me."

Castle scrapes a hand through his hair, looking truly troubled for the first time in the months since they met, and nods. "Fine, but not here."

"Where?"

"I've got an apartment-"

"No," she scoffs, but he raises a hand in supplication.

"If I was going to hurt you, do anything that you didn't want - aside from kill people - then I would have by now," he points out. "So either trust me, or don't. Up to you, Kate."

She huffs, hating when he uses her first name, trying to make it personal. It's not, it… it's just different.

He begins to walk past her, his shoulders shrugging. She curses him, curses them both, and grabs his arm. Promptly lets it go the second the sparks flicker.

"How far is it?" she questions, nodding towards her car, turning on her heel. But he remains immobile in the middle of the alley. "What's the matter, _Rick_? Don't trust me."

The corner of his mouth twitches and he strolls forward.

"I trust you with all of my dark, sociopathic heart," he grins, walking side by side with her towards the Crown Vic.

* * *

Kate stays true to her word, disabling the tracking in her police vehicle for the night, driving straight to his apartment in the Bronx without contacting anyone, swatting at Castle's hand every time he reaches for the radio dial instead.

The apartment she follows him up to is a bare - one bedroom on the second floor, a lot of empty space with a couch in the living room area, a mattress on the floor with rumpled sheets and a few naked pillows strewn across it. A bit depressing.

"Cozy, I know," he murmurs as he shuts the front door. "Can I get you something to drink? Water? A beer?"

"This isn't a social call, we had a deal," she reminds him, taking a seat on his couch and brushing her fingers over the holster still adorning her waist. "Sit. Now."

"You're so bossy," he grumbles, following her to the sofa nonetheless and plopping down beside her. "It's oddly arousing."

"Stop flirting with me."

"Why? Enjoying it too much?" he smirks, whining when she reaches forward to pinch his ear. "Oww, apples."

"Start talking, Castle," she mutters, propping her elbow atop the back of the couch while he relaxes back into the cushioning.

She could shoot him right now, achieve justice for those he's murdered, potentially save future lives. His guard is down, she could end it so easily.

Her hand remains immobile in her lap.

"I had a daughter."

Kate goes almost painfully still, the shock of the statement flushing through her veins like ice.

"You… a daughter?" she echoes, the foreign sensation of empathy for him cracking through her chest at the sad twist of his lips.

"Unbelievable, right?" he chuckles, the sound so gut-wrenchingly hollow. "But yeah, she was… Alexis was my world."

Kate braces herself for the rest, for the reason why he now refers to his daughter in past tense.

"She was - she was only eleven and to this day, I still can't wrap my head around it, why anyone would want her dead," he murmurs, a faraway look claiming his eyes, dulling their shine to a pale grey. "I know I'm biased, but she was damn near perfect. Perfect grades, friends, personality. Perfect daughter who had my heart in her hands. But maybe that was why all along."

"Jealousy?" Kate murmurs, resisting the urge to reach out, cover the ivory knuckles of his fist atop his thigh. Those hands have strangled women to death; she's not touching them.

"It was my ex-wife," he confesses, releasing a shuddering breath. "I had been out of town for the weekend and I didn't want to leave her, but Gina swore to me that she could do it. She reminded me how she'd stayed overnight with Alexis before and assured me that it would be fine. But when I got home the next night… Alexis had come down with meningitis. I still have no idea how, when - but she was sick," he recalls quietly, the devastating mixture of anguish and past concern flushing his face. "It was just some sniffling before I left, a little headache I thought some children's Tylenol would treat, and after that, she seemed fine, normal. She seemed fine before I left," he whispers, swallowing hard. "Just 48 hours and I - Gina hadn't even known, didn't - didn't even _check_ on her. Wouldn't even take my daughter to the hospital. Just let her die, just-"

"Shh, Rick," Kate breathes, turning her back on her morals, the boundaries firmly between them for just that moment. Just long enough to grab his hand as the tears trail down his cheeks.

"I found her. I found her in her bed when I got home. I thought - fuck, I thought she was just sleeping." His voice cracks and so does her heart. "But she was so cold," he rasps, his adam's apple bobbing with another rough swallow he forces down, his jaw squaring harshly as he grapples with his composure. "Manslaughter. That's what Gina was charged with. She never even shed a tear for my kid," he grates out. "If you wanted a sociopath, maybe you should have looked into that case."

"Castle-"

"I kill women like her," he gets out. "Yeah, they're blonde like she was, but that's not the only quality I make sure they share. I study them, watch them, make the decision after a few days."

"You…" Kate's brow furrows as she reviews the files in her head, the victims, their kids. Or lack thereof. "They were all neglectful mothers."

He nods, clasping both hands between his knees and bowing forward. "Neglectful mothers, abusive ones, are my general _type_. I don't just select random, innocent women to kill. I punish those who deserve it."

"But Castle-"

"I know," he growls, shaking his head. "It's wrong and I have no right to decide who lives and who dies, I heard the speech the first time, Kate. And I - I know this isn't what Alexis would have wanted, I know all the rights and wrongs, but after I lost her I just couldn't… I didn't handle it the right way."

"There is no right way to handle grief, but killing people… Rick, you needed help."

"I needed justice," he croaks, sitting up to stare back at her with eyes that burn, bleed with grief. Oh, how she knows the feeling. All too well. "I hadn't planned to turn into a monster, I hadn't even planned to kill Gina, it just - after she got off with no more than a slap on the wrist, showed up at my loft, the home where my daughter lived and died, I - I saw red."

"But after Gina?"

He sighs, sounding so very tired, that charismatic killer she's come to know fading away. He's just a man who lost his daughter, was pushed into choosing the wrong path just as easily as she could have been.

"It wasn't intentional, wasn't trying to become some vigilante," he mumbles. "And it's never been some enjoyable, sadistic act either. I just - started noticing women who reminded me of Gina, of her behavior, and it just escalated from there."

"Would you ever… stop?"

Rick meets her eyes, holds her gaze. "Like I've told you, I knew it was wrong, but I justified it by telling myself that human beings like that didn't deserve to live."

"And now?" she murmurs.

"I can't do it anymore." His shoulders slump a little, the lines of his face seeming to deepen before her eyes. "I'm tired, Kate. And this isn't who I want to be. Never was."

They remain silent for a long moment, her hand still covering his fist. Kate sucks in a breath.

"Why me?" she demands, the need for an answer clawing through her chest. "You evaded the police for years, could have evaded me and my team just as easily, but you allowed yourself to get caught after killing for the first time in…"

"Six years," he supplies on a sigh.

"Why did you make it so personal with me? First, it was just taunting, but then you let me connect evidence to you. May as well have planted it."

"I tried to confess."

"Evidence wasn't substantial enough, idiot."

His lips quirk and hers do too. Even though they shouldn't, even though the idea of him trying to do the right thing softens her, even though it makes her wish - for a lot of things.

"I did make it personal. Not intentionally in the beginning, that was just me having fun annoying you."

She huffs and draws her hand back, but he catches her fingers, brushes his thumb to her knuckles before letting her go.

"But then, you just…" Castle shifts, uncomfortable for the first time too. It feels like meeting an entirely different man, an idea of who he was before he let his tragedy define him. "You made me want to stop. I saw the way you were with the victims' families, your drive, compassion, and it made me feel guilty for the first time. Made me wish that I'd… met you some other way, that I'd been someone else. That I could be better."

Okay, so maybe not a sociopath after all. Just a very good actor.

"Honestly, I'd hoped you would catch me sooner," he murmurs, copying her position and propping his elbow atop the couch, his head against his hand, mirroring her. "But let's face some truths other than mine here, Detective Beckett."

"No idea where this is going, but okay," she replies, arching her eyebrow in challenge, ignoring the sinking sensation in her stomach.

"You've had the chance to shoot me before this, to collect evidence, and you've had ample opportunity to do both tonight. If you wanted me dead, I'd be dead. If you wanted me incarcerated, I'd be well on my way to wearing orange right now."

Kate is silent beside him, her lungs constricting. All of her words are dead on her tongue, because she doesn't have an answer for him, can't even explain it to herself.

"So, tell me, Kate," he says, his eyes clear, blue, and seeking. "Why am I sitting in my living room instead of a prison cell?"

She shakes her head, pushing up from the couch to get away from him, get out of here. If she isn't going to arrest him, she sure as hell isn't going to indulge his desire for her company. Even if it is a shared interest, a shared attraction.

"Beckett-"

"You have to pay for your crimes, you have to make it up to those families, I have to-"

"It's not your responsibility," he argues softly, but she snarls at him.

"You _made it_ my responsibility, you selfish bastard. You lost your daughter and as hellish as those women you killed may have been, whatever they may have done, they were daughters too. You broke apart families, you did everything I work to prevent, and I hate people like you, people who don't think or care for the consequences of their actions. I hate you, I hate you-" She's crying, her eyes burning with tears she refuses to let free, because she just can't do this anymore. Can't handle the conflict of interest, the exhausting push and pull of her heart wanting him while her mind, her morals, condemn her for it. She can't handle caring about someone she's supposed to hate.

Castle stands reaching for her even as she chokes on the lump in her throat and halfheartedly shoves at his chest.

"I hate you," she grits out, shaking her head but swaying towards him. She could never hate him as much as she hates herself for what she's about to say. "But I don't – don't want you to be gone."

The words are raw on her tongue, their first time leaving her lips. She relents in the tentative embrace of his arms, clutches the front of his shirt in her fingers.

"You could still visit me in Rikers," he whispers. She huffs through a sob, a laugh, presses her forehead to his clavicle.

"Stop."

"Then tell me how to fix it, how to - to start making amends, something," he sighs, his hands so unsure at her back, but his palm rubs soothingly between her shoulder blades. "I was better than this before, I can be better again, Kate. For you, I swear I can be better."

A trembling sigh leaves her lips as she lifts her head, touches the tips of her fingers to his cheek. "Castle, I don't think-"

He turns his head to graze his lips to her palm, steals her breath in such a horribly cliché way, but she can only lean in closer. She uses the hand on his face to turn his cheek, tilt her chin, and claim his mouth.

It's wrong, the worst thing she could ever imagine doing, but as Castle's arms wrap around her waist and her body arches so effortlessly into his, she can't recall a time something in her life ever felt so wonderfully right.

* * *

"Yo, Beckett. You open your envelope?" Esposito asks, approaching her desk and cutting his eyes to the manila package on the edge. She casts a disinterested look to the mail and shakes her head.

"No, why?"

"Because you, me, Ryan, and Montgomery all got one," he states, his expression solemn. "They're all from an anonymous sender and they've all contained the same thing."

Her brow dips with confusion and Beckett drags her attention away from the last of her paperwork. She snags the envelope, slits it open with her thumbnail.

She can feel her face blanch at the images inside, barely noticing the sheet of paper that flutters to the surface of her desk. Her gaze is riveted to the photos of Rick Castle, slumped against a wall with a bullet hole in his temple and blood staining his face, neck, and clothing.

"The note says it was for the victims' families, that the debt has been paid," Esposito murmurs. Kate nods, setting the pictures down and taking a deep breath, regaining the neutral expression of her features. "I'm glad the bastard's dead. Got what he deserved."

"Yeah," Kate mumbles, plucking the typed note that had been inside.

 _The debt has been paid; it's been made right._

She sighs and joins Esposito and Ryan in the captain's office, shutting the door and shoving Castle from her mind. She's a detective, he was a criminal; she has no reason to feel remorse.

* * *

The drive to her father's cabin is quiet, tinged with the beginnings of winter, its first snowfall, and a sorrow she just can't shake. Those photos of him dead are playing like a loop through her mind. The sight of him covered in blood, slumped against a concrete wall that not even the precinct's tech analysts could identify, rattling her so harshly her hands shake.

Her body burns for him, cold like the brisk December air, cold since the last time he touched her, yearning for the strike of his hands like a match to her flesh.

She was touched by the hands of a murderer, caressed by fingers that strangled the life out of others, but resurrected hers. Everything changed that night, _he_ changed, and at the altar of her body, it was as if he was made anew, vowing repentance, seeking forgiveness in her skin.

She isn't the one who should give it, isn't the one he so deeply sinned against. But she took his confessions, deemed him worthy of absolution anyway.

Beckett closes her eyes during her brief stop at an intersection, breathes out through her nose, and wills herself to forget. But memories of that night in his apartment are seared into her brain just as easily as his fingerprints branded her skin.

She should be ashamed, drenched in it, and she was, even before she succumbed to their physical attraction, but she can't be anymore. She doesn't want to be.

She's not a believer in divine entities, in the universe or fate, soulmates, but there's something about Rick Castle that filled her with a sense of _right_ when all she has faced and felt and overcome has been a continuous influx wrong since her mother was murdered.

Wrong men, wrong dreams, wrong life.

He can't fix her, but he made it better and that was worth something to her. Worth fighting for.

Kate parks the car in front of the cabin minutes later, checking over her shoulder out of habit as she exits her cruiser and climbs the steps to the porch. She unlocks the front door, flicking the front room light on and breathing in the scent of forest and smoke, the crisp air of winter, and the scent of his aftershave.

"Rick?"

She waits a beat, her heart in her throat and the air caught in her lungs-

Until he emerges from her childhood bedroom, his chest expanding beneath the thin stretch of his t-shirt in relief. Hers does too.

Those pictures of him were staged, the makeup and prosthetics done by a guy he knows from "back in the day", and all constructed without her so not to influence her reaction. The moment she opened the envelope, felt the all too real cinch in her chest and tear in her lungs, she knew why. The second Montgomery gave them the weekend off, she drove straight here, straight to the source of proof to his survival.

Kate strides forward and hooks her arms around his neck, lets herself hold and be held like she did that first night six weeks ago. The night prior to the decision they made.

She feels something for him, has to love him to some degree to go this far to save his life, however wrong or twisted it may be. And while it doesn't make up for his crimes, he's trying every day to repent for his sins, to be a better man. To be worthy of her.

Castle skates a kiss along the slash of her cheekbone before pulling back to see her face.

"Did the families get the photos? Do they - feel a tiny bit better knowing their daughters' killer is dead?"

She sighs, but nods her head. "To some, it will never make a difference, the person they love is gone, but to others… there was vindication for them in seeing their daughter's murderer dead, stopped for good."

"I am," he states, his voice grave and his eyes piercing through hers. "Stopped. I'll never kill again."

"I still believe you," she affirms, and she does. After he's worked his ass off proving it to her in just the past six weeks, after he's sworn to continue proving it. It's his only option.

She's not some naive girl in love with the dashing criminal. He knows she'll shoot him dead if he gives her a reason.

"And we'll make this work, Kate. As long as you want it to, even if it's long distance, we'll find a way," he murmurs, trying so very hard to be positive, hopeful, even though she knows that one day it will come down to him and her job, likely relocating to an entirely new area just to be with him out of hiding.

When it gets to that point, her love for him that strong, then it will be worth it. For now, they live in secret, their love a dark, tragic thing, tended to between the shadows. And every day she is reminded how much easier, logical, and right it would be to stop the madness, to either make those pictures a reality or simply cut this cord of connection between them.

But she can't, she won't.

"I'm sorry, Kate," he whispers at her lack of response. "I'm not – I'm sorry I'm not what you deserve-"

"No," she shushes him, using the drape of her palms at his nape to draw his mouth down to hers, humming at the bittersweet flavor of sorrow and desperation on his lips. "I chose this."

Castle snakes his arms around her waist, bruising a kiss to her mouth, soothing them both with the stroke of his tongue and the brush of his knuckles to the edges of her spine.

"For the record, that night," he mumbles, nipping at her bottom lip. "I thought the physical attraction would be enough to get us out of each other's systems." Kate groans, rising into the press of his body against her, the sizzle of flames beneath the surface of her skin overwhelming, spreading like wildfire when he backs her into the front door. "I was wrong."

"Still hate you," she gasps, rocking her hips into the cradle of his, clawing at the shirt on his back when his mouth opens hot and wonderful over the throb of her pulse.

"I know," he murmurs, cupping her face in his hands, sealing the words to her lips. "Hate how much I love you too, Kate."


End file.
